1. Field of the Invention
The present invention relates generally to heave stabilized floating structures and, more particularly, to floating platforms.
2. Description of the Prior Art
A floating structure, for example, a drilling/production platform, is effectively a spring mass system. As such, it has a resonant (natural) frequency and is subject to resonant oscillatory heave in response to wave and tidal action in the seaway. Resonant motion occurs when the natural period of heave is substantially equal to the period of the wave which induces such heave in the platform.
The patent literature describes various structures and arrangements for dynamically and passively damping a floating platform.
For example, the systems described by Bergman in U.S. Pat. No. 4,167,147, employ arrangements to create anti-heave forces that are in phase opposition and proportional to the heave velocity of the platform (Newtonian damping). These anti-heave forces are much smaller than the actual wave forces which produce the heave.
A platform can be designed so that its natural resonant period T.sub.n occurs at some given wave period T.sub.n, and so as to experience a low resultant vertical force or heave in response to all waves with substantial energy in the design seaway. The design seaway will have a natural heave period T.sub.n, which is greater than the longest period of the wave with substantial energy.
However, because determination of the worst expected or design seaway is based on historical records and statistics, a certain degree of uncertainty can be expected. Therefore, designers are always faced with a remote but real probability that the longest design wave period T.sub.n may be exceeded during the expected life of the floating platform.
Also, the platform's heave displacement is a particularly serious problem for rigid production risers which are suspended by mechanical tensioning devices having a fixed stroke range.